Exchange and Transport Part Four

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Movement of Water up Stems: The main force that pulls water up stems is the evaporation of water from the leaves by a process called Transpiration. Water evaporates through tiny pores called Stomata. Movement of water out through the stomata. When the stomata are open, water vapour molecules diffuse out of the air space into the surrounding air. Water lost from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells. By changing the size of their stomatal pores, plants can control their rate of transpiration. Water is replaced by water reaching the mesophyll cells from the xylem by either the apoplastic or symplastic pathways. In the case of the Symplastic pathway water movement occurs because: Mesophyll cells lose water to the air spaces. These cells now have a lower water potential and so water enters by osmosis from neighbouring cells. The loss of water from these neighbouring cells lowers their water potential. They, in turn take in water from their neighbours by osmosis. There are two main factors that are responsible for the movement of water up the Xylem: Root Pressure and Cohesion-Tension. The Cohesion-tension theory. Operates as follows: Water evaporates from the leaves as a result of transpiration. Water molecules from hydrogen bonds between one another and hence tend to stick together. This is known as Cohesion. Water forms a continuous, unbroken pathway across the mesophyll cells and down the Xylem. As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata more molecules of water are drawn up behind it as a result of this cohesion. Water is hence pulled up the xylem as a result of transpiration. This is called the Transpiration pull. Transpiration pull puts the xylem under tension i.e. there is a negative pressure within the xylem, hence the name cohesion-tension theory. There are several pieces of evidence that support the cohesion-tension theory: During the day when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem, causing the diameter of the tree trunk to increase. At night, when transpiration is at its lowest, there is less tension in the xylem, causing the diameter of the tree trunk to decrease. If a xylem vessel is broken and air enters it, the tree can no longer draw up water. As the continuous column of water is broken and the water molecules can no longer stick together. When a xylem vessel is broken water does not leak out, instead air


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